Problem: On the first 4 quizzes of his physics class, Christopher got an average score of 79. What does he need on the next quiz to have an overall average of 81?
Let his score on the next quiz be $x$ The sum of all of his scores is then $4 \cdot 79 + x$ The same sum must also be equal to $5 \cdot 81$ Solve: $x = 5 \cdot 81 - 4 \cdot 79 = 89$.